There is not currently a 64bit version of N++ (well, offically at least, I think somewhere you can find a 64bit build). There is a page here describing briefly about large files. That being said…there are people very interested in 64bit builds and people have been looking into it. If you dig around on the forum a bit you can find some discussions.

Also, what kind of file is it that it won’t open (XML possibly)? I can open plain text files several hundred megabytes.